In 1970, H. B. Bartl et al., stated that in a 12CaO.7Al2O3 crystal, two oxygen atoms of 66 oxygen atoms in a unit cell having two molecules do not belong to the network, but exist as “free oxygen” in a cage in the crystal (H. B. Bartl and T. Scheller, Neuses Jarhrb. Mineral., Monatsh. 1970, 547).
Hosono et al., of the present inventors discovered that 12CaO.7Al2O3 crystal clathrates O2− ion radicals at a concentration of about 1×1019 cm−3 as measured by electron spin resonance. The 12CaO.7Al2O3 crystal was synthesized by subjecting a mixed material containing CaCO3 and Al2O3 or Al(OH)3 to a solid phase reaction at a temperature of 1200° C. in air. Hosono et al., proposed a model in which part of the free oxygen exists in the cage in the form of O2− ion radicals (H. Hosono and Y. Abe, Inorg. Che. 26, 1193, 1987, “Materials Science”, Vol. 33, No. 4, pp. 171-172, 1996).
The present inventors subjected a mixed material containing calcium and aluminum at an atomic equivalent ratio of about 12:14 to a solid phase reaction under a controlled atmosphere and temperature, whereby a 12CaO.7Al2O3 compound clathrating active oxygen at a high concentration of 1020 cm−3 or more was found. The present inventors filed patent applications of the compound itself, a method for production thereof, a measure for taking clathrated ions, a method of identification of active oxygen ion radicals, and an application of the compound (Japanese Patent Application Publication No. 2001-49524=Japanese Unexamined Patent Application Publication No. 2002-003218, PCT/JP01/03252=WO 01/79115 A1).
The present inventors synthesized a C12A7 compound containing OH− ions at a concentration of 1021 cm−3 or more by hydrating the powder of the 12CaO.7Al2O3 compound in water, a water-containing solvent, or a water vapor-containing gas and firing it under an oxygen atmosphere, and filed patent applications of the compound itself, a method for production thereof, a method of identification of OH− ions, and an application of the compound (Japanese Patent Application Publication No. 2001-117546).
As to the mechanism by which the C12A7 compound includes the active oxygen ion radicals at a high concentration, the present inventors consider that the free oxygen in the C12A7 compound is reacted reversibly with the oxygen molecules entering the C12A7 compound. As to the mechanism by which the OH− ions are included at a high concentration, the present inventors consider that the free oxygen in the C12A7 compound is reacted reversibly with the water molecules entering the C12A7 compound. These mechanisms are described below in detail.
FIG. 1 is a schematic view showing the crystal structure of 12CaO.7Al2O3. 12CaO.7Al2O3 is a cubic crystal system (lattice constant=11.97 angstrom) belonging to a space group of 143d, including two formula of C12A7 per unit cell, and having a melting point of 1449° C. The crystal has a structure in which Ca2+ ions are coordinate with a network structure where tetrahedrons of AlO4 are aggregated. There is a cavity, i.e., a cage, in the crystal lattice.2(12CaO.7Al2O3)=Ca24Al28O66=[Ca24Al28O64]4+.2O2−O2− ion is called free oxygen, and exists in the cage. In general, O2− is always coordinated in the form of a cation in a solid structure, and does not exist in the free state. However, the O2− ions in the cage of the 12CaO.7Al2O3 crystal cannot be combined with the cations, and exist in the free state. This state is similar to the case that the ions are adsorbed on a solid surface, and is considered to be very chemically active.
The O2− ions clathrated in the cage are prevented from directly reacting with the external atmosphere. However, when the temperature increases, the case size becomes large due to thermal expansion. Oxygen molecules in the atmosphere can pass through a bottle neck of the cage, to be thus clathrated in the cage. As a result, the reaction proceeds as follows:O2−(in the cage)+O2(in the cage)=O−(in the cage)+O2−(in the cage)
Through this reaction, the C12A7 compound clathrates a large amount of active oxygen ion radicals.
Two On− ion radicals are produced from two oxygen ions O2− in the unit cell. The 12CaO.7Al2O3 compound clathrating On− ion radicals at a high concentration is described as [Ca24Al28O64]4+.(2−m)O2−(2m)On−. In this formula, m≦2, and On− ion radicals and O2− ions are clathrated in the cage. The oxygen molecules in the cage and in the atmosphere are equilibrated with the oxygen molecules in the atmosphere in a high temperature region in which the oxygen molecules can pass through the bottle neck of the cage. In general, when the temperature increases, the oxygen atoms in the C12A7 decrease in accordance with the Boyle-Charles law. Accordingly, it is considered that the number of active oxygen ion radicals decrease at a higher temperature.
When the synthesized C12A7 contains H2O, the reaction of O2− (in the cage)+H2) (in the cage or in the interstices of the crystal lattice)=2OH− (in the cage) proceed, thus clathrating OH− ions in the cage. The 12CaO.7Al2O3 compound clathrating OH− ions at a high concentration is described as [Ca24Al28O64]4+.(2−m)O2−(2m)OH−. In this formula, m≦2, and OH− ions and O2− ions are clathrated in the cage. The H2O molecules in the cage are equilibrated with the water vapor in the atmosphere in a temperature region in which the H2O molecules can pass through the bottle neck of the cage.